A flight plan is the detailed description of the route to be followed by an aircraft within the framework of a scheduled flight. It comprises notably a chronological sequence of waypoints described by their position, altitude and overflight time. The waypoints constitute the trajectory to be followed by the aircraft with a view to best complying with its flight plan. This reference trajectory constitutes a valuable aid both to the control personnel on the ground and also to the pilot, for anticipating the movements of the airplane and thus ensuring an optimum safety level. The flight plan is commonly managed on board civil airplanes by a flight management system that will be referred to as the FMS subsequently, which notably makes the reference trajectory available to the personnel aboard and available to the other onboard systems.
With a view to safety essentially, it is therefore necessary to ensure that the airplane follows at least in geographical terms and possibly in timetable terms the reference trajectory described in its flight plan. For this purpose, guidance procedures make it possible at the minimum to slave the airplane in space to the 3D trajectory corresponding to the reference trajectory. But before formulating a guidance instruction, it is necessary first of all to ensure that the flight segments ahead of the airplane in the short-term and medium-term can actually be overflown, for example that their overflight is compatible with the performance of the airplane and can be done in compliance with the safety standards. Firstly, it is appropriate to verify that the scheduled route is indeed continuous ahead of the airplane, that is to say it does not exhibit any gap. Specifically, a flight plan can exhibit route discontinuities, notably just where it is envisaged that the flight be performed according to manual flight or fly by visual flight rules. But it also happens that waypoints are deleted without an alternative being given by the pilot or else that the system fails to replace them automatically. In all these cases, of course, it is impossible to establish guidance instructions. Secondly, it is appropriate for example to verify that the horizontal trajectory ahead of the airplane does not impose angles of roll while turning or load factors that the airplane would not be able to withstand. Specifically, the actual conditions may be different with respect to the moment when the turn was defined theoretically, for example in terms of wind. It is also appropriate to verify that the trajectory ahead of the airplane does not require excessive climb or descent slopes, or else excessive or too low speeds that the airplane would not be able to follow, or indeed which would imperil the safety of the flight. All these checks must be made so that discrepancies that are prejudicial both at the performance level and at the safety level do not get passed on late.
So, robustness problems arise with the trajectory data, the latter becoming invalid or poorly suited to the least unforeseen thing. Since in addition to the numerous cases of trajectory discontinuity, the reference trajectory often does not take account of the current specificities of the airplane, since this is not its job. Specifically, the reference trajectory has been calculated in advance when formulating the flight plan, by making assumptions not only about continuity but also about nominal flight conditions of the airplane in terms of performance, flight limits or meteorological conditions. These assumptions often lead to temporary inaccuracies during the flight, or indeed to inconsistencies.
First of all, these assumptions represent ideal external conditions, notably the meteorological conditions, since the latter are hard to forecast with a high confidence level for flights which have to take place in several days. Thus, the actual flight conditions often turn out to be conspicuously different from the forecast conditions, notably in terms of wind to which an aircraft is very sensitive. The wind being variable with altitude and generally being manifested as gusts, the resulting discrepancies are moderate but very changeable.
Thereafter, these assumptions represent a normal operational situation of the craft throughout the flight, the flight model having been supplied with the nominal performance corresponding to the type of craft. But in the event of a fault impacting the actual performance of the airplane, a fault with an engine or a control surfaces fault for example, the airplane is no longer capable of flying its profile such as described in the flight plan on the basis of nominal performance. Its performance is degraded and is specific to the fault or to the combination of faults that have arisen. The case of old airplanes whose performance is degraded because of wear to their engines may also be mentioned. The discrepancies due to degraded performance can be significant, but they are often stable over time.
Finally, it is also possible to envisage the case where the system used to formulate the flight plan does not have a sufficiently up-to-date database at its disposal. It then uses a default performance in its calculations, yet which performance does not quite correspond to that of the craft actually used. Specifically, mention should be made of the low update rate of the databases in these systems.
Current FMS systems seek neither to anticipate nor to correct these problems since this would lead to complex and unwieldy calculations. They display the reference trajectory with possible discontinuities and without verifying, notably during manual flight, whether the airplane is capable of flying the trajectory ahead of it, the airplane then operating in a mode based on notions of target holding, the time to regain a continuous and flyable trajectory. The pilot copes with anything unforeseen in the immediate future by modifying the behavior of the airplane by virtue of the piloting commands. Consequently, the airplane does not exactly follow the reference trajectory extracted from its flight plan when the latter comprises discontinuities. In certain cases it may even diverge considerably therefrom. If appropriate, it is paradoxically the reference trajectory which is updated so that it reflects the actual trajectory, thus making it possible to place the airplane back on a scheduled route at least in the short-term. In a context of fairly loose air traffic management, it is possible to be satisfied with such a situation, notably by virtue of the anticollision alert systems of the “Traffic Collision Avoidance System” type, which make it possible in parallel to ensure short-term safety. But the same is no longer so with the most recent navigation rules, which impose or will impose the closest possible compliance not only with the 3D route filed in the flight plan, but also with the timetables. Compliance with timetables improves safety, but above all optimizes cost through better management of the fleet and infrastructures related to air traffic such as airports for example. The aircraft separation constraints become very significant in areas where, in parallel with this, the traffic is highly concentrated. It is therefore appropriate to follow as closely as possible the reference trajectory described in the flight plan.